Powder sampling method and apparatus

ABSTRACT

A powder sampling method and apparatus is provided for sampling powder from a mixer or a drum. An undisturbed column of powder is extracted from a vessel. The column of powder may then be sectioned into subunits, using a discharge device, providing up to dozens of samples per insertion. Larger number of representative samples of controlled size from a powder bed can be acquired.

RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 U.S.C. § 371 of PCTApplication No. PCT/US00/33529 filed Dec. 11, 2000, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 60/169,929 filedDec. 10, 1999, the entire disclosures of which are expresslyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus forsampling powder, and moves specifically to a method and apparatus forpowder sampling which obtains undisturbed powder samples.

2. Related Art

Powdered materials are used in a wide range of industries. For example,an estimated 80% of pharmaceutical products take the form of tablets,which are compacts of powders. Powdered ingredients such as starches,flour, and sugars are essential raw materials in the food industry.Inorganic powders, such as oxides, nitrides, and carbides are used asraw materials in the ceramic industry. Detergents, abrasives, cosmetics,fertilizers, catalysts, etc., also involve processing powderedcomponents.

Achieving homogeneous and well-characterized blends of powders andgranules is a critical step in the manufacture of pharmaceuticaltablets. Ineffective powder blending can result in increased variabilityin the contents of potent components in tablets, often resulting inrejection of finished product due to poor quality. If mixinginhomogeneities could be identified and/or avoided during themanufacturing process, fewer batches would be rejected, thus reducingmanufacturing costs for existing products and perhaps decreasingtime-to-market for new products.

At the present time, blending of granular materials is largely an artrather than a science. The ability to design and accurately assess amixing process for a high potency drug is limited. Recognition of thisproblem has recently resulted in lawsuits and in tightening of FDAregulations. The situation is complicated by the lack of effectivetechniques for characterizing powder mixtures. In fact, the state of theart in sampling procedures (the thief probe) is often so inaccurate thatit is possible for a high quality batch to be rejected due entirely tosampling error. Poor sampling capabilities have resulted in a lack ofrigorous quantitative evaluations of actual powder mixing processes,further hindering both process development and quality control.

Characterization of homogeneity in a powder system is usually attemptedby taking and analyzing discrete samples. The most common approach instationary powder systems is to use a thief probe to withdraw samplesfrom different locations. Thief samplers belong to two main classes,side-sampling and end-sampling. A typical side-sampling probe has one ormore cavities stamped in a hollow cylinder enclosed by an outer rotatingsleeve. The sleeve has holes that align with the cavities, allowingadjacent powder to flow into the cavities. An end-sampling thief has asingle cavity at the end of the probe; such cavities can be opened andclosed in a controlled manner. In both cases, the thief is introducedinto the powder with the cavities closed. Once insertion is complete,the cavities are opened, allowing the powder to flow into them. Thecavities are then closed, and the thief is withdrawn, removing samplesfrom the mixture.

In principle, the homogeneity of the mixture may be statisticallyestimated from these samples. However, this estimate is meaningful onlyif the probe itself does not introduce errors. As mentioned above, thisis not always the case. Errors are often introduced both when the theifprobe is inserted into the powder bed and when powder flows into thethief cavities. In any sampling scheme, the experimentally measuredvariance, σ_(e) ²,is actually a combination of the true varianceresulting from the mixing process, σ_(m) ², the variance introduced bysampling error, σ_(s) ², and the variance resulting from chemicalanalysis, σ_(a) ². In addition, for granular materials, any sample iscomposed of a finite number of particles, and there is a residualirreducible variance σ_(r) ² i.e.,σ_(e) ²=σ_(m) ²+σ_(s) ²+σ_(a) ²+σ_(r) ²  (1)In an ideal situation, σ_(s) ², σ_(a) ² and σ_(r) ² are negligible, andσ_(e) ² (the variance subject to USP rules) is almost identical to σ_(m)² (the true variance). Unfortunately, thief probes often biasmeasurements to the point that sampling uncertainty is a large fractionof the measurement. Thief probes often introduce two types of errors:(i) the mixture is extensively disturbed when the thief probe isinserted into the powder bed, and (ii) particles of different size flowunevenly into the thief cavities. As a result of such errors, ahomogeneous mixture can be deemed inadequate due entirely to samplingerror.

Accordingly, what is needed, but has not heretofore been provided, is apowder sampling tool that preserves the homogeneity of the mixture andminimizes disturbance of the powder bed.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a thief probe systemto render undisturbed samples in far more accurate and convenient mannerthan conventional thief probes.

It is another object of the present invention to provide a thief probesystem that minimizes disturbance of a powder bed while obtainingsamples.

It is an additional object of the present invention to provide a powdersampling method and apparatus which utilizes a cylindrical tube with asharp or pointed circular lower edge for obtaining samples whileminimizing disturbance of the powder.

It is a further object of the present invention to provide a powdersampling method and apparatus includes a cap for retaining powder withinthe sampler during retraction of the sampler from powder.

It is a further object of the present invention to provide a powdersampling method and apparatus which includes a mechanism for removingpowder samples from the sampler.

The powder sampler of the present invention comprises a cylindrical tubewith a lower end wherein the walls of the cylindrical tube taper to asharp circular edge. The tube can be inserted into a powder bed tocapture a powder sample within the tube. The sample remains within thetube based on arching, or alternatively, a cap can be used to retain thepowder in the tube. The sample can then be removed from the tube inincrements for testing. A powder extraction apparatus for removing thepowder from the tube is included. The extraction apparatus comprises apush rod interconnected with threaded rod such that rotation of thethreaded rod pushes the push rod through the cylindrical tube to pushout the powder sample.

BRIEF DESCRIPTION OF THE FIGURES

Other important objects and features of the invention will be apparentfrom the following Detailed Description of the Invention taken inconnection with the accompanying drawings in which:

FIG. 1 is a perspective view of the powder sampling apparatus of thepresent invention.

FIG. 2 is shows the powder sampling apparatus of FIG. 1 inserted into apowder.

FIGS. 3A, 3B and 3C show an embodiment of the apparatus of FIG. 1 with acapping fixture.

FIG. 4A is a top plan view and FIG. 4B is a side plan view of a coredischarge fixture for removing a powder sample from the powder samplingapparatus.

FIG. 5 is a graph of core sample profiles of the interface betweenAvicel and lactose in a layered system.

FIG. 6 shows a plurality of sampling tubes extending through a templatefor taking a plurality of powder samples.

FIG. 7 shows an experimental set-up for taking a plurality of powdersamples from a blender.

FIG. 8 illustrates the concentration profile for black sand versus depthfor each core sampler.

FIG. 9 displays the axial profile after 30 revolutions

FIG. 10 shows nearly complete homogeneity is achieved after 240revolutions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system for sampling powder from avessel. The device may be herein referred to as a “core sampler.” Asdepicted in FIG. 1, the core sampler of the present invention, generallyindicated at 10, comprises a cylindrical tube 20, having an upper end 22and a tapered lower end 24 which is tapered to a sharp angle 26. Thecylinder 20 has an inner wall 28 and an outer wall 29. At the sharpangle 26, the inner and outer walls 28 and 29 meet. The core sampler 10could be constructed from stainless steel, but other strong metals orpolymers would be equally suitable. To gather a sample, the core sampler10 is inserted into a powder bed to a predetermined depth, thusisolating a cylindrical core of powder within the tube 20. As shown inFIG. 2, by using this approach, a nearly undisturbed column of powder isisolated inside the core sampler 10.

Most materials of interest in pharmaceutical applications aresufficiently cohesive so that they do not drain out of the core sampler10 under gravity. A simple cylindrical tube 20 is all that is requiredto pull a column of such powder from the bed. Cohesive powders resistflow due to gravity; they stick within the cylindrical tube 20 due toparticle-particle and particle-wall frictional forces which lead to“arching.” Since the frictional forces of cohesive powders are high,cohesive powders are prone to arch. Arching will occur more readily insmaller diameter tubes, since the arching distance is decreased, and theratio of tube surface area to volume of powder within the tube diameteris increased. Therefore, to increase the likelihood of extracting asample of powder, smaller diameter core samplers may be used.Importantly, the core sampler of the present invention can be used totake vertical samples, samples at an angle, or even horizontal samples.Also, it should be noted that the inner diameter of the core sample canbe varied in accordance with the powder being sampled and the size ofthe sample desired.

However, if smaller diameter cores are not practical, or for extremelyfree flowing powders, the core can be used with a capping fixture, asshown in FIGS. 3A, 3B and 3C. In this configuration, a fixture 40designed to cap the end is attached to the exterior of the tube 20 bymeans of eyelets 42. This capping fixture 40 has two components, (i) aplow rod 44 with a sharp conical plow 46 affixed at one end and aremovable plow handle 48 on the opposite end, and (ii) a cap tube 50with a thin circular disk or cap 52 affixed at one end and a removablecap handle 54 on the opposite end. The practice of using the coresampler 10 comprises of three steps. The starting configuration isdepicted in FIG. 3A. The cap tube 50 is slid through the eyelets 42 onthe tube 20 and the cap handle 54 is attached at a marked position. Theplow rod 44 is then slid within the cap tube 50 and the plow handle 48is attached. In the starting position, the cap 52 rests flush on top ofthe plow 46. In step 2, the core sampler 10 is inserted into a powderbed, generally indicated at 11 to a predetermined depth, as shown inFIG. 3B, thus isolating a cylindrical core of powder within the tube 20.In step 3, the open end of the core tube 20 is capped so that the powdercore remains in the tube 20 during extraction of the sampler from thepowder bed 11. To accomplish this, the plow rod 44 and cap tube 50 arefurther extended into the powder bed 11 to a level where the cap 52 isjust below the level of the core tube 20. The cap tube 50 is thenrotated so that the cap 52 covers the end of the core tube 20. Once thesampler is capped, it is pulled from the powder bed, as shown in FIG.3C. Importantly, the plow 46 and cap 52 do not interfere with the samplewithin the tube 20. Rather, the cap and plow 52 are moved to below theopening of the tube 20 after the sample is already in the tube 20,thereby eliminating disturbance of the sample by the plow 46 and cap 52.

The present invention also includes a reliable and robust method fordischarging the core sample. Since it is desirable to subdivide thepowder column into a number of undisturbed samples of controlled weight,the discharge fixture must withstand very large forces sometimesrequired to move cohesive powders through small diameter cores. The coredischarge fixture is generally indicated at 60 in FIGS. 4A and 4B. Thefixture 60 comprises a tube holder 62 having an upright flange 63attached to a metal base 64. The upright flange 63 has an apertureslightly larger than the cylindrical tube 20 for receiving and holdingthe cylindrical tube 20. Locking screws, not shown, may be used toretain the cylindrical tube 20 securely in place during sampleextraction. A push rod 70 is inserted into the cylindrical tube 20 topush the powder sample therefrom. The push rod 70 is driven through thecylindrical tube 20 by a ram comprising a sliding platform 74. The pushrod 70 is mounted on the platform 74 by mount support 72. The platform74 is mechanically coupled to a linear motion system comprising aprecision-threaded rod 76, a handwheel 78, guide rails 80, and collars82 and 84. One collar 82 links the sliding platform to theprecision-threaded rod 76, while the other collars 84 connect thesliding platform to the guide rails 80. The linear motion system isattached to the base 64, so no relative motion occurs between thecylindrical tube 20 and the linear motion system. The guide rails 80 areinterconnected with the base 64 by attachment to pillow blocks 81. Whenthe handwheel 78 is rotated, the precision-threaded rod 76 turns, movingthe sliding platform 74 forward, sending the push rod 70 linearlythrough the cylindrical tube 20. Sample size is controlled by the numberof turns on the handwheel 78. In cases where the material is freeflowing, the core discharge fixture 60 can be held at angle ofinclination greater than the complement of the angle of repose of thepowder. This insures that powder is discharged due to the action of thedischarge device and not from gravitational flow.

The performance of the core sampler was evaluated using a layered systemof microcrystalline cellulose (Avicel, FMC Corporation) and granulatedlactose. The lactose was dyed red in order to distinguish it visuallyfrom microcrystalline cellulose, which is white. The powders wereclassified by sieving. The Avicel exhibited a particle size less than 90microns. The lactose exhibited a particle size ranging from 500 to 710microns, nominally 600 microns. A layered system of lactose on top ofmicrocrystalline cellulose was formed in a 2000 ml beaker with adiameter of approximately 4 inches. The Avicel layer was 4 inches thick;the lactose layer was 2.5 inches thick.

The performance of three core samplers with different size innerdiameters, ⅞ inch, {fraction (11/16)} inch and {fraction (7/16)} inch,were evaluated. The core samplers were used in the manner described inthe previous section. The volume of the collected sample may beapproximated by the inner diameter of the tube multiplied by thedistance which the screw ram is driven between collections. Bycontrolling the number of rotations of the screw driven ram, the size ofthe collected sample and therefore the number of samples collected fromeach core was varied. Typically the ram was extended between 0.2 inchesand 0.4 inches between samples, corresponding to samples weighingbetween 0.4 grams and 2 grams. Highly uniform sample weight was achievedby monitoring the sample weight during discharge from the core. Sampleweight variability was much lower (relative standard deviation=2%) thanfor the thief probes described above.

FIG. 5 demonstrates the ability of the core sampler to accuratelyprofile the interface between Avicel and lactose in a layered system. Inthese cases, the extracted core of powder was divided into approximated20 smaller sections. The composition of the sections was determined bysieve separation (recall the Avicel had a much smaller particle size).The interface between the Avicel and lactose is sharply resolved, withlittle contamination of Avicel in the lactose layer or vice versa. Theaxial resolution of the technique was determined to be less than 1centimeter. That is, the contamination of powder from one stratum to thenext is limited to less than 5% after a distance of 1 centimeter. Incontrast, other commercial thief probes yield a much smaller number ofsamples, require similar or larger amount of labor, and exhibitsignificantly greater contamination.

To describe the uniformity of powder within a large vessel it is oftennecessary to sample many locations. The core sampler of the presentinvention is efficient at extracting a column of powder from the vessel,with tight axial resolution, but it is also necessary to take samples atdifferent radial positions. Insertion of the core sampler into thepowder bed does disrupt the powder outside of the sampler, especially incases where the plowing and capping rod are used. It is necessary totake into account the size of the disrupted zone of powder in order todetermine the minimum lateral separation between sampling locationsrequired for accurate sampling.

The disrupted zone of powder can be visualized using layered systems.Most of the disruption is caused by the insertion of the plowing andcapping rod. On the side of the capping and plowing rods, the disruptedzone of powder extends for up to 5 centimeters from the core. On theside opposite of the capping and plowing rod, the disrupted zone ofpowder extends less than 1 centimeter from the core. When the cappingand plowing rods are used (as would be the case for free flowingpowders), a conservative recommendation for the minimum recommendedseparation between sampling locations is 5 core diameters. When thecapping and plowing rods are not used (as in most cases) the minimumseparation between sampling locations is 2 core diameters.

Removal of the core sampler causes significant disruption to the powderbed, because powder outside the sampler collapses into the gap left byremoval of the core, disrupting powder several centimeters away.Therefore, in order to maintain the highest amount of radial resolution,an adequate strategy for sampling is to insert all of the core samplersinto the powder vessel prior to removing any of them. In this manner,the disturbances to the powder bend during removal of the core cannotaffect the powder isolated within the other cores.

As an example, core samplers were used to describe the characterizationof blend homogeneity in a 2 cubic foot Tote-Blender manufactured by GEIGalley. This device is an asymmetric tumbler with the bottom sectionshaped as a hopper and the top a rectangular box. The axis of rotationdoes not bisect the blender into two equal halves but rather a skewedpartition in order to break the symmetry of rotation.

White and Black Art Sand from Clifford W. Estes Co., Inc., located inLyndhurst, N.J. was mixed in the tote-blending experiments. The range ofparticle size was between 0-500 μm for both colors. Although the sandused in these experiments was free flowing, the core samplers with thecapping rod in place did not yield enough samples, necessitating the useof simple cores. A successful sampling strategy was implemented asfollows:

(1) At the end of the mixing experiment, the lid of the blender wasremoved, and a sampling grid, was attached to the top of the blender tomaintain consistency in spacing and straight passage through thegranular bed as shown in FIGS. 6 and 7.

(2) Core samplers were introduced into the mixture without removing anyof them.

(3) Tests in layered system showed that the infiltration process did notdisturb the sand within the cores.

(4) The samplers were removed discharged in a controlled manner tosubdivide them into multiple samples.

The cores used had an inner diameter of 0.75 inches and the rods wereinserted 3.5 inches apart. A total of nine rods spanned the spacedallotted by the opening of the blender.

The sampling procedure and the time spent on each step are providedbelow for the case of one operator performing the sampling alone.

1. Fix the sampling grid on the tote-blender opening upon completion ofa mixing experiment (1 minute).

2. Insert the core samplers into the grid holes making sure the passthrough the sand bend until their bottom contact the inner hopper walls(1 minute).

3. Extract each core sampler, take it to the discharge device, andextrudate the sample from each sampler, dividing it into “unit does”samples. This procedure takes 9 minutes.

4. The next step is to process the individual unit-dose samples using anappropriate method of analysis. Time required for this procedureobviously depends on the chemical nature of the material and thereforewas not estimated for a general case.

The outcome of this sampling procedure is concentration data. FIG. 8illustrates the concentration profile for black sand versus depth foreach core sampler. The initial condition for this experiment had theblack and white sand loaded side by side, each on a different side ofthe baffle, filling 40% of the total volume of the vessel. FIG. 9displays the axial profile after 30 revolutions. As it is the case forall tumbling mixers, axial mixing is slow; the regions that hadinitially one color of sand remained high in concentration of that colorfor this particular experiment. Cores 7, 2, 6 are placed in theinitially black sand region (positive x-axis), while cores 8, 4, 9 arein the initially white region (negative x-axis). Cores 3, 1, 5approximately sample the division line between the colors as shown inFIG. 7. The entire set of 106 values is reported in Table 1. Meancomposition measured by the procedure is 50.44%, in excellent agreementwith the amount of black sand initially loaded to the vessel. The RSD is49%, demonstrating that the mixture is still far from homogeneous after30 revolutions. As shown in FIG. 10, nearly complete homogeneity isachieved after 240 revolutions.

In summary, it must be concluded that samples obtained usingconventional thief probes are likely to contain significant errors. Theinsertion of a thief probe into a mixture causes extensive disturbancesof the mixture structure, dragging particles along the path of insertionof the thief. The sample that is finally collected is likely to containparticles from all positions along the path. Even in the best case,samples were contaminated by particles originally located as far as 5 to10 cm away from the sampling location causing errors of 10% or more,i.e., considerably larger than desirable for an accuratecharacterization of mixture structure.

The device of the present invention enables the extraction of anundisrupted column of powder from a vessel. The column of powder maythen be sectioned into subunits using a specially designed dischargedevice. This allows one to render a large number of representativesamples from a powder bed. The core sampler is able to resolve theinterfacial layer with an axial resolution of less than 1 centimeter.The radial resolution is demonstrated to be equal to the diameter of thecore. Therefore, the described sampling technique is a significantlymore accurate means to extract undisturbed powder samples from anintended location. Additionally, the technique is more efficient and maybe used to render a larger number of samples with less labor. Also, theweight of the collected samples is controllable. Given these advantagesover conventional powder samplers and thieves, the sampling devicedescribed here is a simpler and better technique for determining contentuniformity of powder within a vessel.

Having thus described the invention in detail, it is to be understoodthat the foregoing description is not intended to limit the spirit andscope thereof. What is desired to be protected by Letters Patent is setforth in the appended claims.

1. A powder sampling apparatus comprising: a cylindrical tube having anouter surface and an inner surface, an upper edge and a lower edge; anangle at the lower edge formed by the outer surface angled towards theinner surface to form a pointed circular lower edge of the cylinder; anda conical plow movable along the exterior of the cylindrical tube and adisk positionable on the base of the conical plow and rotatable withrespect to the conical plow to cover the lower edge of the cylindricaltube.
 2. The apparatus of claim 1 wherein the conical plow isinterconnected with a rod, and the disk is interconnected with a sleevepositioned about the rod, the rod and sleeve attached to the cylindricaltube, the disk rotatable from a first position sitting on the plow to asecond position covering the lower edge of the cylindrical tube.
 3. Theapparatus of claim 2 further comprising a rod handle interconnected withthe rod for driving the plow, and a disk handle interconnected with thesleeve for rotating the disk.
 4. The apparatus of claim 3, wherein thedisk handle is rotatable with relation to the rod handle to move thedisk.
 5. The apparatus of claim 2, further comprising a plurality ofeyelets for retaining the rod and sleeve along the cylindrical tube. 6.The apparatus of claim 5, wherein the rod and the sleeve are attached toextend parallel to the cylindrical tube.
 7. The apparatus of claim 2,wherein the rod and the sleeve are rotatable with respect to each other.8. The apparatus of claim 2, wherein the disk is positioned at a wideend of the conical plow.
 9. A powder sampling apparatus comprising: acylindrical tube having an outer surface and an inner surface, an upperedge and a lower edge; an angle at the lower edge formed by the outersurface angled towards the inner surface to form a point about the loweredge of the cylinder; and a conical plow interconnected with a rod, anda disk interconnected with a sleeve positioned about the rod, the rodand sleeve attached to the cylindrical tube, the disk rotatable from afirst position sitting on the plow to a second position covering thelower edge of the cylindrical tube.
 10. The apparatus of claim 9 furthercomprising a rod handle interconnected with the rod for driving theplow, and a disk handle interconnected with the sleeve for rotating thedisk.
 11. The apparatus of claim 10, wherein the disk handle isrotatable with relation to the rod handle to move the disk.
 12. Theapparatus of claim 9, further comprising a plurality of eyelets forretaining the rod and sleeve along the cylindrical tube.
 13. Theapparatus of claim 12, wherein the rod and the sleeve are attached toextend parallel to the cylindrical tube.
 14. The apparatus of claim 9,wherein the rod and the sleeve are rotatable with respect to each other.15. The apparatus of claim 9, wherein the disk is positioned at a wideend of the conical plow.